TOKYO—Japanese voters headed to the polls Sunday to decide who would fill half in the upper house of parliament in a contest that is largely seen as a gauge of the public's reception to the economic policies of Prime Minister Shinzo Abe.

Going into the House of Councilors election, the race is predicted to hand the ruling Liberal Democratic Party a majority. Mr. Abe's cabinet enjoys a near 60% approval rating on the back of support for his economic policy mix of monetary easing, fiscal spending and pro-growth steps....